Geochemistry of Metal and Metalloid Thioanions

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HelzEAR-0229387This project will explore the geochemistry of selected aqueous thioanions. Thioanions consist of a metal or metalloid cation coordinated at least partially by sulfide ions. The chemical behavior of these anions in nature has received little study. The information obtained could have applications in such diverse areas as managing solid (including nuclear) wastes, improving our understanding of ore formation, reconstructing the past redox status of the ocean and healing coastal ecosystems.Using previous studies of Mo behavior in sulfidic waters as a prototype, we will explore mechanisms that trigger particle-association of Re. This element is of interest in paleoenvironmental studies, in geochronology and as a laboratory model of the nuclear waste component, Tc. In oxic environments, the stable form is strongly hydrophilic ReO4-. In suboxic to anoxic environments, Re becomes particle-associated. Mechanisms that could trigger this change in behavior include transformation to thioperrhenates or reduction to ReIV complexes. The nature of the products formed and equilibrium constants characterizing such changes are the objective of this portion of the project.Thiomolybdate ions have been observed to dissolve very insoluble Fe minerals, like pyrite. Formation of strong Fe-Mo-S complexes is implied. We will study this phenomenon systematically in order to characterize the dissolved complexes. If such complexes are capable of reversible equilibrium with pyrite, then they may catalyze formation of this mineral from Fe monosulfide. Mechanisms of pyrite formation remain an area of active interest (and some dispute) among geochemists, despite considerable previous work. Our discovery of very strong complexing between CuI and the oxythioarsenite ion, AsS(OH)(SH)-, has sparked interest from theorists and led to predictions of similarly strong thioarsenite complexes involving other potentially hazardous metals (e.g. Pb2+, Hg2+). We will test these predictions. Complexes of this kind result in synergistic co-transport of hazardous metals and metalloids. They should be accounted for in computational models used for designing hazardous waste sites and making decisions about disposal options for dredge spoil, mine tailings, etc.